Intracluster interactions in butterfly {Fe(3)LnO(2)} molecules with the non-Kramers ions Tb(III) and Ho(III).

The intracluster exchange interactions within the “butterfly” [Fe(3)Ln(mu(3)-O)(2)(CCl3COO)(8)(H2O)(THF)(3)] molecules, where Ln(III) represents a lanthanide cation, have been determined by a combination of x-ray magnetic circular dichroism (XMCD) and vibrating sample magnetometry (VSM) along with an interaction model. We have studied the compounds with Ln = Tb and Ho, both non-Kramers lanthanides and with high uniaxial anisotropy, and Ln = Lu(III) and Y(III) as pseudolanthanides, which supply nonmagnetic Ln reference cases. At low temperature, the three Fe atoms can be considered as a self-unit with total spin S-Fe3 = 5/2. Using the element selectivity of the XMCD magnetometry, measured at the Ln L-2,L-3 edges, together with the VSM measurements, the local magnetization of the Ln ion and the Fe-3 subcluster, as a function of the field and low temperature (T approximate to 2.5 K), has been determined separately. These results are described quantitatively in the framework of a theoretical model based on an effective spin Hamiltonian, which considers the competing effects of intracluster interactions and the external applied magnetic field. The Ln-Fe-3 exchange interaction within the {Fe(3)LnO(2)} cluster has been determined to be antiferromagnetic, in both Tb and Ho compounds, with J(FeTb)/k(B) = -0.13(1) K and J(FeHo)/k(B) = -0.18(1) K, respectively. In both cases, a field-induced reorientation of the Fe-3 and Ln spins from antiparallel to parallel orientation takes place at a threshold field H-mu 0 = 1.1 and 2 T, for the {Fe3TbO2} and {Fe3HoO2} compounds, respectively. By comparison with other compounds of the series with uniaxial anisotropy, it is concluded that the polarizability of the Fe-3 subcluster magnetic moment decreases in the trend {Fe3YO2} -> {Fe3TbO2} -> {Fe3HoO2} -> {Fe3DyO2}, because of the increasing opposition of the exchange antiferromagnetic field caused by the Ln ion. In the Ln = Tb, Ho, and Dy, the magnetization of the whole molecule is dominated by the anisotropy of the Ln ion. The intracluster Fe-3-Ln exchange interactions are very weak compared to the Ln ligand field and Fe-Fe exchange interactions.